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Coach stitching is usually pretty straight and consistent (stitch size), Coach isn't perfect, and it's not uncommon to see some irregularity. I don't see problems with the stitching in the pictures you've shown.
I'm not sure what the "separation" you're referring to at the bottom of the zipper means, but if you're referring to the two halves beneath the stop, that's on every zipper you'll find that has a closed end. A zipper is made of two strips of binding, each side with teeth. In order to have the zipper stop and not fall apart when open, they use a metal clamp at the end and the zipper will only open as far as that clamp. Below the stop, there are the 2 pieces of binding. There can be slight variation in the amount of binding that shows below the stop but there's always some small amount that shows. The only way to prevent that would be for them to sew across the stop, thus both breaking needles and also preventing the zippered opening from opening fully.
The last picture shows overstitching or backtacking. That's done to prevent the stitching from unraveling and you'll find it on bags, clothing, and anything else that's sewn.
I don't see the fraying you're referring to.
The bag is absolutely authentic. You can take comfort in knowing you got what you paid for -- an authentic Coach bag.